Aluminium alloys



Patented Dec. 8, 1936 i I UNITED STATES PATIENTOFFICE-1],;

Adolf Beck, 132mm, cuni filmoi mi. G.

Farbenindnstrie Aktiengesellsc Frankforton-the-Main, Germany No Drawing.Application May 31, 1934, Serial No. 728,383 ,In Germany June 9, 1933 4'Claims. (01. 148-211) I This invention relates to a process of improvingas manganese or silicon may be present in the althe resistance tocorroding agents'as also to deloys whether deliberately added or not,their caterioration' of their mechanical properties when pability offorming compounds with magnesium, in contact with corroding agents, ofaluminium however, being taken into account when the quan- 5 base alloysof the kind hereinafter set forth, tity of magnesium to be incorporatedin the al- 5 v The aluminium base alloys to which the invenloy iscomputed, in order always to ensure the tion relates, contain magnesiumin an amount not presence of available magnesium in the alloy. onlysufilcient to combine with any further al- In aluminium base alloyswhich contain availloying constituents presentwhether deliberately ablemagnesium andwhich have been caused to added or notbut also in excess ofthat required solidify from the molten or fused state by allow- 10 toform atroom temperaturea saturated solid soing them to cool in theordinary manner, the lution with the basic aluminium. The amount ofmicrographic structure is found to consist of two magnesium which isthus in excess of that remain constituents or phases, namely of largeaquired for forming asaturated solid solution with crystals which ineach case consist of an unthe basic aluminium in a state uncombined withsaturated solid solution of the compound AlzMgs 15 any foreignconstituents present will be hereinin aluminium, and of acoarse networkof smaller after referred to as available magnesium. particles of p-typecrystals surroundingthe said.

A further essential constituent of the alloys to a-crystals and,probably consisting of a solid soluwhich the invention relates is zinc,the latter metal tion of minor quantities of aluminium in the whichcombines with magnesium more readilycompound, AlsMgz. However, thetexture thus 20 than aluminium, being also present in an amount formedis not in phase-equilibrium and it is in excess of that required to format room tempertherefore possible by. an annealing process to esature asaturated solid solution with the basic tablish the actual equilibriumby intercrystalline aluminium. Such amount is hereinafter referreddiffusion in amanner known per se and,- as apto as available zinc. Sincethe limit or soluplied to alloys of the system Al-Mg. already de- 25bility, at room temperature, of the compound H scribed by Hansom andGaylor in the Journal of AlsMgz corresponds to a content of about 3% ofthe Institute of-Metals XXIV (1920) pp. 201-232. magnesium of the alloy,and further, since the In this manner, the e-crystals are enabled tolimit of solubility of the MgZnz compound at the take up furtherquantities of AhMgP-compound same temperature is attained with a contentof from the intercrystalline network upto theirilimit 30 about of ma sand p ndingly of saturation, and, by applying sufliciently high about1.0% of zinc, and since the presence of temperatures-ranging betweenabout 250 and both compounds simultaneously in an aluminium about-450 C.and depending on the magnesium base alloy only moderately affects theirrespective content of thealloy-for a sufllcient length of solubilitylimits, this means in effect that the al-. time, the whole texture ofthe alloys is reduced to 35 'loys contemplated by the present inventionmust an entirely homogeneous mass of u-crystals which contain more thanabout 3.2% of magnesium and are more or less saturated with respect tomagnemore than about 1.0% of zinc, the ratio between slum, according tothe percentage of the latter these two components being invariably suchthat element which is presentin the alloy. 7

40 there is more than about 3% of magnesium still' As a result of recentand hitherto unpublished 40 available for combination withthe aluminiumresearch, it has been ascertained that by subbase metal after all thezinc present has comjecting the homogeneous alloys consisting of binedwith some of the magnesiumpresent. Thus, supersaturated u-crystals to anannealing proawhen in accordance with the present invention ess attemperatures which are lower than thos'e I the alloys are subjected to aheat treatment as at which they were previously homogenized, but 4:5 I

hereinafter set forth, supersaturated solid soluwhich are at the sametime sufliciently high to tions of both magnesium and zinc in aluminiumallow of intracrystalline segregation (i. e. within are formedby reasonof the increased solubility a moderate temperature range from about 2001 of both these metals in aluminium in the solid C. upwards), themagnesium content of the a- 5 state, with rising temperatures, fromwhich both crystals exceeding the limit of saturation at that I theavailable magnesium (which is combined with temperature is caused tosegregate from within aluminium) and the available zinc (in combinathea-crystals in a highly dispersed form; that tion with anotherpart of themagnesium) may be is to say, every individual a-crystal becomesinsegregated in the form ,ofthe compounds, reterspersed withra largenumber of minute parspectively,Alal\igzandMgZnz. Other metals, suchticles of the p-type. This terture of course is 55 again heterogeneous,but is distinguished from the heterogeneous texture of the alloys ascast, in that the p-type constituent is no longer coarsely distributedin a network surrounding the acrystals, but is highly dispersed in auniform manner throughout and within the said a-crystals; when viewedwith the microscope, a texture of this kind, after the application ofsuitable etchingagents offers the mottled appearance which is well knownas characteristic of a segregate in a highly dispersed form.

Further research, upon which the present in vention is based, hasrevealed the fact that the heating and annealing treatment causing firsthomogenization of the alloys containing only available magnesium, andthen segregation of the compound AlsMBz therefrom in a highly dispersedform, when applied to alloys as defined in the introductory paragraphsof this specification, simultaneously causes also segregation of theavailable zinc in the form of the compound MgZna the resulting productsnot only exhibiting a very high resistanceto corrosion e. g. byseawater, but-also exceptionally high mechanical strength values. I

The term corrosion" as, employed above does not only apply to anexternally and macroscopically manifest corrosion, for example bydiscoloration of the surface of the metals, but particularly also tointercrystalline corrosion phenomena which in course of time lead to adecrease in the mechanical strength properties. This intercrystallinecorrosion is particularly conspicuous in the alloys after their havingbeen subjected to a plastic deformation in the cold, for instance by aforging or rolling process. According to present day views suchintercrystalline corrosion is in fact considered far more dangerous froma technical point of view because of the lowering of the mechanicalstrength values of the material produced thereby; and its progress istherefore usually followed by determining the progressive decrease intensile strength etc. after progressively prolonged contact oi thealloys with corroding media.

In accordance with the present invention the alloys of the kinddescribed in the introductory paragraphs of this specification aresubjected to a thermal treatment which consists inilrst heating thealloys to temperatures sumcient to cause the alloying elements magnesiumand zinc to be incorporated in-the aluminium base crystals in the 'formof a solid solution and maintaining such temperatures until ahomogeneous solid solution has been substantially established and then,in a second stage, which may be preceded by a chilling, annealing thealloys attemperatures at which and until segregation of the compoundMgZnz as well as of particles which are rich in magnesium as comparedwith the basic crystals Example 1 An alloy extruded into sections, andcontaining 6% of magnesium, 3% of zinc,-and 0.5% of manganese'and thebalance aluminium, and in which consequently about 0.5% of magnesium isrequired to combine with the zinc so asv to form the compound MgZna, andthe available magnesium is about 2.5%, in the original state, had

V the following strength properties:-

Ultimate tensile strength kg/sq.mm 32.9 Elongation per cent; 26.3 Yieldpoint kg/sq.mm 13.2 Contraction per cent 42.8

Ultimate tensile strength kg/sq. mm 45 Elongation per cent 17 Yieldpoint kg/sq.mm 28 Contraction per cent 50 The resistance to corrosion ofthe alloy in question was tested by immersing highly polished test rodsin seawater for 3 months and then examining their strength properties inthe usual manner, the results being that no decline of the originalstrength values of the test pieces could be ascertained.

Example 2 Another extruded alloy containing about 6% of magnesium, about4% of zinc and 0.3% of manganese, about 0.7% of magnesium thus be-' ingrequired to combine with the zinc and the available magnesium beingabout 2.3%, in the griginal state, had the following strength properiesrUltimate tensile 'strength kg/sq.mm 39 Elongation per cent 12.5 Yieldpoint kg/sq.mm 21.2 Contraction per cent 24.6

This alloy was subjected to the same thermal treatment as described inExample 1. The strength properties of the improved alloy were found tobe':

Ultimate tensile strength "kg/811mm 54.2 Elongation per cent 10.1 Yieldpoint kg/sq. mm 51.2 Contraction per cent 35.0

This improved alloy also showed no decline in its strength propertieswhen exposed to a 3 months attack by sea-water and then tested as above.A

The alloys according to the invention lend themselves to extruding,sheet rolling and forging. The range of alloys particularly adapted tothe treatment according to the invention covers about'4 to 12% ofmagnesium and 2 to 6% of zinc,,while minor percentages of other alloyingmetals such as manganese may be added. Silicon may also be present insmall quantities whether deliberately added or not.

I claimz- 1. The process of increasing the resistance to corrosion aswell as the strength properties of an aluminium base alloy containingmagnesium 7 in quantities of between about 3 and about 12 percent andzinc in quantities of between about 2 and about 6 percent, theproportion of magnesium to zinc being such as to leave more than 3percent of magnesium, after the formation of the compound MgZnz with allthe zinc present, in a state available for combination with aluminium,which comprises subjecting the alloy to an annealing treatment attemperatures between about 400 and about 440 C. for a period of timesufficient to convert the crystalline structure of the alloysubstantially into the form of a homogeneous solid solution, andthereafter subjecting the alloy to a lower temperature, but above about200 C., so as to cause segregation of the compound MgZnz and of thecompound AlaMgz in a highly dispersed form.

2. The process of increasing the resistance to corrosion as well as thestrength properties of an aluminium base alloy containing magnesium inquantities of between about 3 and about 12 percentand zinc in quantitiesof between about 2 and about 6 percent, the proportion of magnesium tozinc being such as to leave more than 3 percent of magnesium, after theformation of the compound MgZnz with all the zinc present, in, a stateavailable for combination with aluminium, which comprises subjecting thealloy to an annealing treatment at temperatures between about 400 andabout 440 C. for a period of time sufficient to convert the crystallinestructure of the alloy substantially into the form of a homogeneoussolid solution, then chilling the alloy, and thereafter heating thealloy to a lower temperature, but above about 200 C., so as to cause atleast partial segregation of the compound MgZnz and of the compoundAlaMgz in a highly dispersed form.

3. In the art of making alloy articles having aluminium as the principalingredient and containing magnesium in quantities of between about 3 andabout 12 percent and zinc in quantities of between about 2 and about 6percent, the proportion of magnesium to zinc being such as to leave morethan 3 percent of magnesium, after the formation of the compound MgZmwith all the zinc present, in a state available for combination withaluminium, the steps of subjecting the article to an annealing treatmentat temperatures between about 400 and about 440 C. for a period of timesuflicient to convert the crystalline structure of the alloysubstantially into the form of a homogeneous solid solution, andthereafter subjecting the alloy to a lower temperature, but above about200 C., so as to'cause segregation of the compound MgZnz and of thecompound AlsMgz in a highly dispersed form.

'4. A heat-treated aluminium base metal alloy containing magnesium inquantities of between about 3 and about 12 percent and zinc inquantities of between about 2 and about 6 percent,

the proportion of magnesium to zinc being such as to leave more than 3percent of magnesium, after the formation of the compound MgZnz with allthe zinc present, in a state available for combination with aluminium,the said alloy being characterized by containing a segregate of bothMgZnz-compound and AlsMgz-compound in highly dispersed form within thebase mass of crystals rich in aluminium.

ADOLF BECK.

